Mask correcting method

ABSTRACT

There is provided a method for correcting a photo mask, which allows the difference between a test mask and a corrected mask with respect to an error of line width depending on coarse/dense pattern to be decreased when the photo masks are corrected by optical proximity effect correction. 
     The present method is consisted of: producing a test mask which acts as a mask for extracting process model for applying an optical proximity effect correction method (s1); transferring and measuring the dimensions of the transferred pattern using the test mask (s2 and s3); obtaining a function model (referred to as process model) of which a simulated result of the transferred pattern of a mask pattern of the photo mask using a function model matches the measured result (s4); obtaining a mask pattern of which a transferred pattern matches a designed pattern using said process model and creating mask data in accordance with the obtained mask pattern (s5); producing a corrected mask in accordance with the created mask data (s5); and setting an exposing condition where an OPE characteristic becomes flat with respect of wide and narrow pitches by adjusting at least one of a numerical aperture (NA) and a coherence factor (σ) of an exposing device when the corrected mask is transferred.

This application claims priority to Japanese Patent Application No.JP2003-037699, filed Feb. 17, 2003 which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a method for correcting a photo maskusing an optical proximity correction (OPC) method, in particular, to amask correcting method for removing as much difference between a testmask and a corrected mask with respect to an error of line widthdepending on coarse/dense pattern.

BACKGROUND ART

As a semiconductor device is being miniaturized and highly integrated, acircuit pattern thereof is being miniaturized. As a circuit pattern isbecoming miniaturized, in an optical lithography process for patterninga semiconductor device, required resolution is becoming high. As aresult, NA of a projection optical system of an exposing device isbecoming large. The wavelength of an exposing light source is becomingshort.

The resolution (R) in the optical lithography is expressed by thefollowing Rayleigh's formula.R=k1×{λ/(NA)}  (1)where λ represents the wavelength of exposure light; NA represents thenumerical aperture of the exposing device; k1 represents a constant thatdepends on the condition and so forth of the optical lithography.

As the required resolution (R) is becoming high, since the k1 factor isbecoming low, it becomes difficult to obtain the desired resolution.

In addition, as the performance and integration of a semiconductordevice are becoming high in recent years, the required resolution isbecoming high. As a result, an optical lithography in a region of whichk1 is small, namely low-k1 lithography is performed.

In the low-k1 lithography, the mask production error of a photo masklargely affects the line width of a pattern (hereinafter referred to ascritical dimension (CD)) of a transferred pattern transferred to awafer.

Thus, in the low-k1 lithography, an inclined light is used for anexposing device. A phase shift mask is used as a photo mask. Inaddition, an OPC (Optical—Proximity Effect Correction) thatbias-corrects the dimensions and shape of a mask and deforms the mask isused so as to correct the optical proximity effect.

In the OPC correction, as will be described later, a process model isextracted from a transferred result of a process model extracting mask.A mask corrected value is obtained in accordance with the process model.A corrected mask is produced in accordance with the obtained maskcorrection value.

As a factor that represents the emphasis rate of the influence of thepattern dimensional error of a photo mask (mask CD error) to thedimensional error of a transferred pattern transferred on a wafer (waferCD error), an MEF (Mask Error Enhancement Factor) expressed by thefollowing formula (2) is generally used.MEF=wafer CD error/(mask CD error/M)   (2)where M represents a reduction projection magnification of an exposingdevice. M is typically 5 or 4 for an exposing device used for producinga semiconductor device.

In the low-k1 lithography used in recent years, the MEF calculated bythe formula (2) for a critical pattern may become 2 to 3. In otherwords, in the low-k1 lithograph, the dimensional error of a pattern of aphoto mask largely impacts the dimensions of a transferred patterntransferred to a wafer. Thus, the influence of the dimensional error ofa pattern of a photo mask against a transferred pattern is becominglarge.

Next, with reference to FIG. 8, a conventional OPC correcting method fora mask will be described. FIG. 8 is a flow chart showing a procedure ofa conventional OPC correcting method for a mask.

First of all, as shown in FIG. 8, at step S₁, a mask for extracting aprocess model, namely a test mask, is produced. The process model is afunction model of which a simulated result of a transferred pattern of aphoto mask using a function model matches a measured result of atransferred pattern of a test mask. Test masks are produced so that testpatterns having various shapes, dimensions, and pitches that represent acircuit pattern of a real device are placed on test masks.

Thereafter, the flow advances to a test pattern transferring step, stepS₂. At step S₂, process conditions such as an exposing condition of theexposing device, a resist process condition, and an etching processcondition are set. A test mask is transferred to a wafer under theprocess conditions that have been set. The transferred test mask isprocessed. The transferred pattern is formed on the wafer.

Thereafter, at step S₃, the pattern dimensions of the transferredpattern formed on the wafer are measured by an SEM (Scanning ElectronMicroscope) or the like. Thereafter, at a process model extracting step,step S₄, a process model is created in accordance with the measuredvalues of the pattern dimensions.

Thereafter, at a corrected mask producing step, step S₅, a correctedmask pattern that allows desired pattern dimensions and pattern shape tobe obtained (extracted) after the test mask has been transferred andprocessed in accordance with the process model. A corrected mask isproduced in accordance with the corrected mask pattern.

In the foregoing steps, a corrected mask that has been corrected by theOPC, namely a product mask, can be produced.

Thus, the OPC correction is performed by steps of production of testmask→transfer→measurement of dimensions→extraction of processmodel→obtainment of mask corrected values→production of corrected mask(product mask). The transferred pattern is measured with the correctedmask. As a result, the corrected mask is evaluated. Thus, the OPCcorrection for a mask is performed by complicated steps.

A conventional OPC correction for a mask is described in for exampleJapanese Patent Application Publication No. 2002-122977, pp 2-3.

In the low-k1 lithography used in recent years, the impact of thedimensional error of a pattern that takes place in production of a maskis becoming very large because a mask pattern should be more accuratelytransferred than before.

On the other hand, it is very difficult to produce a mask pattern of aphoto mask in accordance with its design. The dimensions of a maskalways contain a dimensional error of a pattern (tolerance). Inparticular, a problem to be solved is dependency of coarse/dense patternor an error of line width depending on coarse/dense pattern.

As shown in FIG. 1, even if target line widths of mask patterns of photomasks are the same, the dimensional error of a pattern of dense lines isdifferent from the dimensional error of a pattern of isolated lines.There is a tendency of which the dimensional error of a pattern of denselines is larger than the dimensional error of a pattern of isolatedlines.

For example, as reported with Proc. SPIE VOL. 4754 (2002) pp. 196-204,“Advanced pattern correction method for fabricating highly accuratereticles”, although the dimensional error of a pattern of a mask mainlyresults from a drawing error of a mask pattern and an etching error atan etching step performed after a patterning step and a developing stephave been performed, dimension fluctuating coarse/dense characteristic,namely an error of line width depending on coarse/dense pattern isbecoming considered.

An error due to drawing in production of a mask is corrected by EB(electron beam) exposure amount correction. An error due to etching of asubstrate performed after a drawing step and a developing step iscorrected by a pattern correction. However, it is difficult tocompletely control the error of line width depending on coarse/densepattern.

As conventional photo mask specifications, there are a means dimensionaltolerance (mean to target) and evenness of line width on a plane.However, the error of line width depending on coarse/dense pattern hasnot been mentioned in a roadmap such as ITRS (International TechnologyRoadmap for Semiconductors).

However, as will be described in the following, an error of line widthdepending on coarse/dense pattern of a mask largely affects atransferred CD error of a transferred pattern.

So far, at an optical lithography step, dimensional errors of patternsof photo masks have not been largely concerned. However, in the low-k1lithography used nowadays, it is considered that two photo masksproduced in accordance with the same specifications have transferred CDsof transferred patterns that largely differ.

This is because a small difference between two photo masks with respectto an error of line width depending on coarse/dense pattern isemphasized by a large MEF of the low-k1 lithography used in recentyears.

A characteristic of a transferred CD in the case that a pattern pitch ofa mask pattern is varied, namely a graph showing the relation between apattern pitch and a transferred CD, is referred to as an OPE (OpticalProximity Effect) curve. This characteristic is one of basic data fromwhich a mask corrected value is obtained.

Thus, an OPE curve as basic data with which a mask is corrected isaffected by both an optical proximity effect and an error of line widthdepending on coarse/dense pattern.

As described above, each of a mask pattern of a test mask and a maskpattern of a corrected mask contains an error of line width depending oncoarse/dense pattern of a mask. Thus, the difference between a test maskand a corrected mask with respect to an error of line width depending oncoarse/dense pattern becomes important.

At the process model extracting step, step S₄, assuming that a test maskhas been produced in accordance with its design, a process model isproduced. However, in reality, a mask pattern of a test mask contains anerror of line width depending on coarse/dense pattern. Thus, since aprocess model that is produced contains a pattern dimension error, anerror of line width depending on coarse/dense pattern of a test maskcorresponding to an error of line width depending on coarse/densepattern of a mask is propagated to a process model.

When a corrected mask having the same error depending on coarse/densepattern as a test mask can be produced, the transferred CD error of thecorrected mask can be decreased to almost zero. However, in reality, thedifference between a test mask and a corrected mask with respect to anerror of line width depending on coarse/dense pattern unavoidably takesplace in the range of a production error. It is known that when acorrected mask is transferred, the difference between the masks isemphasized by the MEF as described above.

Thus, the difference between an error of line width depending oncoarse/dense pattern of a test mask and an error of line width dependingon coarse/dense pattern of a corrected mask, namely the differencebetween masks, affects the machining dimensions of the pattern of whichthe corrected mask is transferred. As a result, an unignorable error,namely, a mask corrected residual error, takes place in the transferredpattern.

As a result, it becomes difficult to obtain a transferred pattern havinga high pattern dimensional control accuracy.

However, in the conventional mask correcting method using the OPC, thedifference between masks is not specially considered. When a processmodel is produced by a test mask and a corrected photo mask corrected inaccordance with the process model is produced, the influence of thedifference between the test mask and the corrected photo mask withrespect to the error of line width depending on coarse/dense patterncannot be removed. The difference between the masks resides as an errorelement of the OPC corrected error. Thus, it becomes difficult toimprove the correction accuracy.

Therefore, an object of the present invention is to provide a method forcorrecting a photo mask, namely a method for managing production of amask that allows the difference between a test mask and a corrected maskwith respect to an error of line width depending on coarse/dense patternto be decreased when the photo masks are corrected by optical proximityeffect correction.

DISCLOSURE OF THE INVENTION

The inventor thought about the following. In other words, to minimizethe difference between a test mask and a corrected mask with respect toline width depending on coarse/dense, a mask having a line widthdepending on coarse/dense that is an intermediate value of a productionerror is used as a test mask.

In addition, in the range of which a process margin for which acorrected mask is transferred does not deteriorate, at least one of NAand a of an exposing device is finely adjusted so that an OPE curve iscontrolled and the difference between masks with respect to an error ofline width depending on coarse/dense pattern is matched. As a result, aresidual error on an OPE curve can be suppressed when a corrected maskis transferred. In this example, a represents a coherent factor and hasa relation of σ=NA of lighting system/NA of object side of projectionlens.

Form the foregoing, the inventor conceived a method for improving thecorrection accuracy of the OPC by finely adjusting one of NA and car ofan exposing device, matching a test mask and a corrected mask, andremoving the difference between the test mask and the corrected maskwith respect to an error of line width depending on coarse/densepattern.

In addition, the inventor confirmed that the obtained conception iseffective with a simulated result and an experimented result. Finally,the inventor invented a method according to the present invention.

To accomplish the foregoing object, in accordance with the foregoingconception, the method for correcting a mask according to the presentinvention is a method for correcting a photo mask using an opticalproximity effect correction method.

The method for correcting a mask comprises the steps of:

producing a test mask that functions as a mask for extracting a functionmodel necessary for applying the optical proximity effect correctionmethod so that a line width depending on coarse/dense is equal to orlower than a predetermined level and is an intermediate value of aproduction error;

transferring a mask pattern of the test mask on a wafer and measuringthe dimensions of the transferred pattern;

obtaining a function model (referred to as process model) that allows asimulated result of the dimensions of the transferred pattern of thephoto mask on the wafer matches a measured result obtained at themeasuring step;

obtaining a mask pattern of which a transferred pattern matches adesigned pattern using the process model and creating mask data inaccordance with the obtained mask pattern;

producing a corrected mask in accordance with the created mask data in aproducing condition of which an error of line width depending oncoarse/dense pattern of the corrected mask matches that of the test maskin a predetermined range; and

obtaining at least one of a numerical aperture (NA) and a coherencefactor (σ) of an exposing device so that an OPE characteristic becomesflat in a predetermined range of a pattern pitch when the corrected maskis transferred.

According to the method of the present invention, at least one of anumerical aperture (NA) and a coherence factor (σ) of an exposing devicethat allow an OPE characteristic becomes flat in a predetermined rangeof a pattern pitch is obtained when a corrected mask is transferred.When the mask is exposed in those conditions, the difference between thetest mask and the corrected mask with respect to an error of line widthdepending on coarse/dense pattern that resides in production of a maskcan be suppressed in a predetermined range.

In the example, when ring-shaped lighting is performed, σ is composed ofinner σ and outer σ. The inner σ is σ inside a ring-shaped light source,whereas the outer σ is σ outside the ring-shaped light source.

At the test mask producing step of a preferred embodiment of the presentinvention, a test mask is produced in a mask producing condition ofwhich an error of line width depending on coarse/dense pattern of thetest mask can be restricted in an allowable range and that the error ofline width depending on coarse/dense pattern is an intermediate value ofthe production error. The mask producing condition of the test mask isobtained through an experiment or a simulation calculation. Thus, a maskof which an error of line width depending on coarse/dense pattern isrestricted in the allowable range and is an intermediate value of theproduction error can be used as a test mask.

At the corrected mask producing step, a corrected mask is produced in amask producing condition of which the difference between the test maskand the corrected mask with respect to an error of line width dependingon coarse/dense pattern is in the predetermined range. The maskproducing condition of the corrected mask is obtained through anexperiment or a simulation calculation. Since mask data is extractedfrom a process model and the corrected mask is produced in the maskproducing condition in accordance with the mask data, most of theinfluence of the difference between the masks can be removed.

In addition, at the exposing condition setting step, the influence ofthe difference between the test mask and the corrected mask with respectto the error depending on coarse/dense pattern is cancelled inaccordance with the error of line width depending on coarse/densepattern of the test mask. Thus, NA and α of the exposing device arefinely adjusted so that the OPE curve of which the corrected mask istransferred becomes the flattest. Thus, the difference between thecorrected mask and the test mask that slightly resides at the correctedmask producing step can be removed.

The basic concept of the method according to the present invention is asfollows.

-   -   (1) To cause the production error of the mask due to        coarse/dense pattern to be lower than an allowable level, an        allowable range of an error of line width depending on        coarse/dense pattern of a test mask is set. In addition, a mask        having an error depending on coarse/dense pattern that is an        intermediate value of the production error is used as a test        mask. Thus, a dimensional error of a pattern propagated to a        transferred pattern on a wafer due to an error of line width        depending on coarse/dense pattern of a test mask can be        restricted in an allowable level.    -   (2) An error of line width depending on coarse/dense pattern of        a test mask is matched with that of a corrected mask in a        predetermined range. Thus, the error of line width depending on        coarse/dense pattern of the corrected mask is reproduced as the        error of line width depending on coarse/dense pattern of the        test mask in the predetermined range. As a result, a dimensional        error of a pattern on a wafer due to the difference between        masks can be restricted in the predetermined range.    -   (3) In (2), although an error of line width depending on        coarse/dense pattern of a corrected mask matches an error of        line width depending on coarse/dense pattern of a test mask in a        predetermined range, the difference between the corrected mask        and the test mask with respect to the error depending on        coarse/dense pattern slightly resides. Thus, at least one of NA        and σ of an exposing device is adjusted so that the test mask        and the corrected mask are matched and the residual difference        between the masks becomes zero. In addition, as the result of        (2), when the residual difference between the masks is removed,        the dimensional error of the transferred pattern on the wafer        can be further decreased.

Next, the foregoing basic concept will be described in detail.

Description of (1)

As described above, when a test mask is produced, a dimensional error ofa pattern takes place depending on whether the pattern is coarse ordense. When a test mask is patterned, the etching rate of quartz thatcomposes a mask substrate in a dense region having dense lines (denseline region) is lower than that in an isolation region having isolatedlines (isolated line region). As a result, as shown in FIG. 1, the linewidth of a pattern in the dense region is apt to be larger than that inthe isolation region.

FIG. 1 is a graph showing a typical relation between a wide/narrow pitchand a production error of a mask. In FIG. 1, a production error of amask is measured in the manner that the line width of the line patternis fixed and the pattern pitch is varied from a narrow pitch to a widepitch.

For simplicity, with reference to FIG. 2, transferred CDs of photo maskshaving a dense pitch (dense lines) and an isolation pitch (isolatedlines) will be described. FIG. 2 is a graph showing a relation between apattern pitch and a transferred CD with a parameter of an error of linewidth depending on coarse/dense pattern. Graph (1) is an OPE graph whenan error of line width depending on coarse/dense pattern ΔM is zero. Inother words, graph (1) represents an optical proximity effect to beextracted. Graph (2) is an OPE graph when a mask having an error of linewidth depending on coarse/dense pattern ΔM is transferred.

In the graphs, the error of line width depending on coarse/dense patternΔM is expressed by the following formula.ΔM=D _(E) −I _(E)where D_(E): production error of mask having dense pitchI_(E): production error of mask having isolation pitch

When the OPE graph (1) and the OPE graph (2) shown in FIG. 2 arecompared, the transferred CD of dense pitch in the case that the errorof line width depending on coarse/dense pattern is ΔM is larger than thetransferred CD of the dense pitch in the case that the error of linewidth depending on coarse/dense pattern is zero by MEFd×ΔM. In otherwords, the transferred CD is emphasized by MEFd×ΔM in comparison withthe case of ΔM=0. In this case, MEFd represents MEF in the case that apattern has a dense pitch.

Thus, since the OPE curve (2) of the test mask having an error of linewidth depending on coarse/dense pattern deviates from the OPE curve (1)of an ideal mask that does not have an error of line width depending oncoarse/dense pattern, the former has an error.

Consequently, when the test mask has an error of line width depending oncoarse/dense pattern, the calculated corrected value of the maskcontains an error. The error is propagated to the dimensions of atransferred pattern of a corrected mask.

FIG. 3 is a graph showing an influence of an error of line widthdepending on coarse/dense pattern of a test mask against a dimensionalerror of a transferred CD of a corrected mask. FIG. 3 shows a simulatedresult of which a transferred CD error of a corrected mask in the casethat the difference between the test mask and the corrected mask withrespect to an error of line width depending on coarse/dense pattern iszero. The simulation is performed in the following conditions.

Mask: half tone phase shift mask having a transmissivity of 6%.

Patterns of test mask

-   -   Ideal mask: isolated lines having a width of 140 nm and dense        lines having a width of 140 nm and a pitch of 260 nm.    -   Real mask: isolated lines having a width of 140 nm and dense        lines having a width of 140 nm+ΔM and a pitch of 260 nm        where ΔM represents an error of line width depending on        coarse/dense pattern of the test mask. As represented by the        horizontal axis of FIG. 3, ΔM is a parameter that varies from −6        nm to +8 nm.    -   Target value of transferred CD: isolated line having a width of        110 nm and dense lines having a width of 110 nm and a pitch of        260 nm.    -   Exposing condition: exposure light/ArF, NA=0.60, σ=0.75, ⅔ ring        shape

A process supposed in the simulation deals with a standard ArF resisthaving a contrast γ of 10 and a diffusion length of 30 nm.

FIG. 4 is a table showing calculated data of the graph shown in FIG. 3.FIG. 4 shows a calculated result of a simulation of transferred CDerrors of corrected masks in the case that errors of line widthdepending on coarse/dense pattern of test masks are varied.

FIG. 3 and FIG. 4 show that even if the difference between the test maskand the corrected mask with respect to the error of line width dependingon coarse/dense pattern is zero, the line width of the corrected maskdoes not become 127 nm (line width of a pattern having a pitch of 260 nmof the corrected mask calculated when the error of line width dependingon coarse/dense pattern of the test mask is zero). Instead, a smallerror takes place in accordance with the error of line width dependingon coarse/dense pattern of the test mask. When the corrected mask istransferred, the small error is emphasized by the MEF. Thus, as shown inFIG. 3, it is clear that an unignorable error resides in the line widthof the pattern transferred to the wafer.

It is thought that when the error of line width depending oncoarse/dense pattern of the test mask is relatively large, the linearityof the mask correcting system is not fully maintained.

Description about (2)

As shown in FIG. 1, since the difference between the test mask and thecorrected mask with respect to the error of line width depending oncoarse/dense pattern is emphasized by the MEF and affects the line widthof the transferred pattern of the corrected mask, it is important tomanage the difference between the masks.

In other words, to produce a corrected mask, it is necessary to managethe difference between the masks based on the error of line widthdepending on coarse/dense pattern of the test mask. In addition, whenthe production error is considered, it is desired that the errordepending on coarse/dense pattern of the test mask should be anintermediate value of the production error.

Description about (3)

FIG. 5 shows a simulated result of transferred CDs of two masks that arean ideal mask (mask A) of which an error of line width depending oncoarse/dense pattern ΔM is zero and a nearly real mask of which ΔM is 2nm on a wafer (mask B). For the masks A and B, a process model isextracted with a test mask of which an error of line width depending oncoarse/dense pattern is zero. As shown in FIG. 9, after the correctedmask is transferred, the line dimensions of the corrected mask iscalculated for each pitch in the condition that the through-pitch is 110nm. FIG. 9 is a graph showing a corrected value of a mask in thethrough-pitch.

Assuming that a mask having an error of line width depending oncoarse/dense pattern ΔM of zero (mask A) and a mask having an error ofline width depending on coarse/dense pattern ΔM of 2 nm (mask B), theirtransferred CDs are calculated. FIG. 10 shows an error of line widthdepending on coarse/dense pattern of the mask B used for the evaluation.

FIG. 5 shows that in the initial state of which the pitch is small,since the error of line width depending on coarse/dense pattern of themask is emphasized by the MEF with respect to each pitch and line width,the OPE curve of the mask B largely deviates from the OPE curve of themask A. It is clear that as the pitch becomes large, namely when thepitch exceeds around 500 nm, the OPE curve of the mask A almost matchesthe OPE curve of the mask B.

When the numerical aperture (NA) of the exposing device is slightlyvaried, for example NA is changed from 0.60 to 0.58, the OPE curve ofthe mask B can be matched with the OPE curve of the mask A.

FIG. 6 shows a graph of a simulated result of which masks are matched byfinely adjusting NA of the exposing device. FIG. 6 shows OPE curves ofthe masks A and B that are exposed with NA=0.59 and 0.58 as well as OPEcurves of the mask A and the mask B that are exposed with NA=0.60.

From FIG. 6, it is clear that when NA is slightly varied, the differencebetween the mask A and the mask B with respect to the error of linewidth depending on coarse/dense pattern can be almost removed.

As described above, in the method according to the present invention,when at least one of NA and σ is slightly varied, the difference betweena test mask and a corrected mask with respect to an error of line widthdepending on coarse/dense pattern can be removed unlike the conventionalmask correcting method using the OPC.

Thus, the dimensional error of a transferred pattern on a wafer due tothe difference between a test mask and a corrected mask with respect tothe error of line width depending on coarse/dense pattern can beremarkably decreased.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a typical relation between wide and narrowpitches and a production error of a mask.

FIG. 2 is a graph schematically showing a relation between a patternpitch and a transferred CD with a parameter of an error of line widthdepending on coarse/dense pattern of a mask, namely a so-called OPEgraph.

FIG. 3 is a graph showing influence of an error of line width dependingon coarse/dense pattern of a test mask against a transferred CD of atransferred corrected mask.

FIG. 4 is a table of data of the graph shown in FIG. 3.

FIG. 5 shows OPE curves of simulated results of transferred CDs of twocorrected masks of an ideal mask (mask A) of which an error of linewidth depending on coarse/dense pattern ΔM of a corrected mask is zeroand a nearly real mask (mask B) of which ΔM is 2 nm.

FIG. 6 shows OPE curves of simulated results of which masks are matchedby finely adjusting NA of an exposing device.

FIG. 7 is a flow chart showing a procedure of a mask correcting methodaccording to an embodiment.

FIG. 8 is a flow chart showing a procedure of a conventional maskcorrecting method.

FIG. 9 is a graph showing dimensions of a mask for which OPC has beenperformed in accordance with the relation of a pitch and a CD of acorrected mask.

FIG. 10 is a graph showing an error of line width depending oncoarse/dense pattern of a mask B (on a wafer).

BEST MODE FOR CARRYING OUT THE INVENTION

Next, with reference to the accompanying drawings, an embodiment of thepresent invention will be described practically and in detail.

EMBODIMENT

The embodiment is an example of a mask correcting method according tothe present invention. FIG. 7 is a flow chart showing a procedure of themask correcting method according to the embodiment.

First of all, at a test mask producing step, step S₁, a test mask isproduced. The test mask is a mask that functions as a mask forextracting a process model corresponding to the optical proximity effectcorrection. When the test mask is produced, an allowable range of anerror of line width depending on coarse/dense pattern is set inaccordance with an analyzed result shown in FIG. 3. Thereafter, a maskproducing condition is set so that the error of line width depending oncoarse/dense pattern of the test mask is restricted in the allowablerange. The test mask is produced in the mask producing condition.

The mask producing condition for which the test mask is produced thesame as a mask producing condition for which the error of line widthdepending on coarse/dense pattern of a corrected mask produced at stepS₅ is restricted in the allowable rang. In other words, it is preferredthat the error of line width depending on coarse/dense pattern is anintermediate value of the production error.

Thereafter, the flow advances to a test mask transferring step, step S₂.At step S₂, a mask pattern of the test mask is transferred to a wafer inthe same transfer condition as a real photo mask.

Thereafter, the flow advances to a transfer wafer measuring step, stepS₃. At step S₃, the dimensions of the transferred pattern transferred onthe wafer are measured by CD-SEM or the like.

Thereafter, the flow advances to a process model extracting step, stepS₄. At step S₄, a function model of which a simulated result of atransferred pattern of a mask pattern of a photo mask using a generalpurpose OPC simulator matches the measured result at step S₃, namely aso-called process model, is extracted or obtained. Thus, the transferredpattern of the mask pattern of the photo mask simulated in accordancewith the process model by the general-purpose OPC simulator becomes thetransferred pattern corresponding to the measured result obtained atstep S₃.

Thereafter, the flow advances to a mask correcting step, step S₅. Atstep S₅, a mask pattern that matches a designed pattern is obtainedusing the process model obtained at step S₄ by the foregoinggeneral-purpose simulator. As a result, mask CAD data for the mask to beproduced is created.

Thereafter, the follow advances to a corrected mask producing step, stepS₅. At step S₅, a corrected mask is produced in accordance with thecreated mask CAD data.

When a corrected mask is produced, it is produced in a mask producingcondition of which an error of line width depending on coarse/densepattern of the corrected mask matches an error of line width dependingon coarse/dense pattern of the test mask in a predetermined range.

Thereafter, the flow advances to a corrected mask transferring step,step S₇. At step S₇, OPE curves are matched by adjusting the exposingcondition of the exposing device.

Since the test mask and the corrected mask are produced in the foregoingmask producing condition, although errors of line width depending oncoarse/dense pattern of the masks match in the predetermined range,since the difference between the masks with respect to the error of linewidth depending on coarse/dense pattern due to the reproducibility ofthe optical lithography process resides, the error of line widthdepending on coarse/dense pattern of the test mask is slightly differentfrom that of the corrected mask.

Thus, at step S₇, by slightly varying the numerical aperture (NA) andcoherent factor (σin, σout) of the exposing device in an allowable rangeof a lithography margin composed of a focus margin and an exposureamount margin, the corrected mask is transferred. As a result, anexposing condition is obtained so that the OPE curve of the transferredCD becomes flat with respect of wide and narrow pitches.

When the corrected mask is transferred in accordance with the obtainedexposing condition, the difference between the masks with respect to theerror of line width depending on coarse/dense pattern can be almostremoved.

According to the present invention, when the corrected mask corrected bythe OPC method is transferred, at least one of the numerical aperture(NA) and the coherence factor (σ) of the exposing device that allow theOPE characteristic to be flat in the predetermined range of the patternpitch is obtained. In the obtained condition, the mask is exposed. As aresult, the difference between the test mask and the corrected mask withrespect to the error of line width depending on coarse/dense patternthat resides in production of masks can be suppressed in thepredetermined range.

When a test mask is produced in the mask producing condition of which anerror of line width depending on coarse/dense pattern of the test maskis restricted in the allowable range, the test mask is used, a processmodel is extracted using the test mask, and the line width of thecorrected mask is calculated in accordance with the process model, theerror of line width depending on coarse/dense pattern of the transferredpattern of the corrected mask can be restricted in the allowable range.

When a corrected mask is produced in the mask producing condition ofwhich the difference between the test mask and the corrected mask withrespect to the error of line width depending on coarse/dense pattern isrestricted in the allowable range, the most of the difference betweenthe test mask and the corrected mask with respect to the error of linewidth depending on coarse/dense pattern can be removed.

In addition, at the exposing condition setting step, at least one of NAand σ of the exposing device is slightly varied so that the processmargin does not deteriorate and when the corrected mask is transferred,a residual error of the OPE curve becomes minimum for all pitches. Thus,when the test mask and the corrected mask are matched on the basis ofthe error of line width depending on coarse/dense pattern of the testmask and the difference between the test mask and the corrected maskwith respect to the error of line width depending on coarse/densepattern is removed, the difference slightly resides between the masks atthe producing step of the corrected mask can be removed. Thus, thecorrection accuracy of the mask can be remarkably improved.

1. A method for correcting a photo mask using an optical proximityeffect correction method, said method for correcting a photo maskcomprising the steps of: producing a test mask; transferring a maskpattern of said test mask on a wafer and measuring dimensions of atransferred pattern; obtaining a model that simulates a result of themeasured transferred pattern of said photo mask on said wafer; obtaininga corrected mask pattern which matches a designed pattern by using saidmodel; and selectively setting an exposing condition to alter at leastone of a numerical aperture (NA) and a coherence factor (σ) of anexposing device depending upon a pattern pitch of a region where saidcorrected mask is transferred.
 2. A method for correcting a photo maskaccording to claim 1, wherein at said step of producing a test mask,said test mask is produced in a mask producing condition of which anerror of line width depending on coarse/dense pattern of said test maskis limited in a predetermined range.
 3. A method for correcting a photomask according to claim 2, wherein at said step of producing a correctedmask, said corrected mask is produced in a mask producing condition ofwhich the difference between said test mask and said corrected mask withrespect to an error of line width depending on coarse/dense pattern isin the predetermined range.
 4. A method for correcting a photo maskaccording to claim 1, wherein at said step of setting the exposingcondition, at least one of NA and σ of an exposing device is adjusted sothat the OPE curve of which the corrected mask is transferred becomesthe flattest over all the pitches, and that the difference between thecorrected mask and the test mask with respect to the error depending oncoarse/dense pattern is reduced.